Energy Measurement Units

The development of energy measurement units dates back to the 18th century. They are used both in everyday life and international energy trade. Some units provide information on the mass and volume of fossil fuel sources. Others help choose the location of renewable energy production sites. Finally, energy itself- produced and consumed in the form of heat or electricity- is measured in specific units.

Why Do We Need Specific Tools to Quantify Energy?

In trade and everyday life, producers, distributors, and consumers have to be able to measure the energy they sell, transport or use according to its characteristics. To do this, they use different energy measurement units adapted to each type of energy (as well as their multiples and sub-multiples, depending on the quantity of energy to be measured).


These units are indespensable to trade, useful to companies in the energy field, and to the following as well:

   • Governments and institutions that need information on energy production and consumption (at national, European or global level) in order to define their energy policies.

Producers, distributors, and consumers have to be able to measure energy.

 
  • Stakeholders in the manufacturing and tertiary sectors (ex. when determining the amount of energy they need to plan overhead for their business)

   • Individuals (ex. when they want to improve their homes to make them less energy-intensive)

Societies created their own measurement units adapted to their economic and commercial needs for hundreds of years. For these historical and cultural reasons, the same units were not always used in different regions of the world. In order to facilitate and standardize scientific research and commercial transactions, physicists in the 18th century established the International System of Units (SI). Managed by the International Bureau of Weights and Measures (BIPM), this system includes widely used units such as meters and kilograms. It also collates a number of specific energy-related units, such as the watt (which expresses electrical power, i.e. the energy transported within an electric system per time unit).

The SI is recognized and used in all countries. However, some energy units that are not part of this system are still in use throughout the world. For example, a barrel is the main means of measuring oil volumes. At the end of the 19th century, American oil companies stored crude extracted from underground in large blue barrels (abbreviated as bbl) and they quantified production in terms of barrels. These powerful companies imposed the barrel as an international unit of measurement. Similarly, the British continued to use imperial units of measurement (dating from 1824) parallel to the SI; such as the cubic foot or ft3, which is equivalent to 0.028m³.

Energy types cannot be compared because each has its own specific features.

False. In order to compare different energy types, scientists have developed a unit of measurement recognized by international organizations - ton of oil equivalent (or ‘toe'). This unit corresponds to the energy produced when a ton of oil is burnt. Since this energy source holds an essential place on a global scale, toe is well suited to assess a country's or region's energy consumption or production. This is why it is often used by economists.

Industry uses the term barrel of oil equivalent or ‘boe' (the energy provided when a barrel of oil is burnt). The lesser used term ton of coal equivalent (‘tce') refers to the energy released when a ton of coal is burnt.

How Do We Measure the Volume and Mass of Oil, Gas, Coal, Nuclear Fuel, and Biomass?

Some units are used to measure a given quantity of natural energy resources- raw materials subsequently used to produce heat or electricity or as fuel for vehicles.

   • The most common unit for measuring the volume of crude oil is the barrel (bbl), which equals 158.98 liters of crude. A refinery's daily output is measured in barrels per day (BPD). But when measuring an oil tanker' cargo of crude, it is expressed in tons (1 t = 1,000 kg) because the shipping cost depends more on the cargo's mass than on its volume. Oil can be very dense and heavy, or less dense and light, meaning that oil with the same volume can have a very different mass- which is more or less expensive to transport. Finally, the fuel in service stations is sold by the liter or by the gallon (UK: 4.546 gal= 1 l. and US: 3.785 gal= 1 l.).

   • The volume of Natural gas is measured in cubic meters (m³) and its multiples. In the United Kingdom, the cubic foot or ft³ is also used. A cubic foot equals 0.028m³. The measurement of a volume of gas takes place under certain physical conditions. This is because the density (i.e. mass per unit volume, expressed in kg/m³) of gas changes according to temperature and pressure conditions.  This is its key from a commercial standpoint- the higher the gas' density, the more hydrocarbon molecules it contains and the higher its energy yield when burnt. This is why gas volumes are always measured at the same temperature and at the same pressure, i.e. 15°C and 750 hectopascals (the hectopascal is the unit for measuring pressure - its abbreviation is hPa). The volume of liquefied natural gas (LNG) is measured in cubic meters, cubic feet or gallons. However, its mass is measured in tons when being transported by LNG tanker (an LNG tanker transports natural gas, 90% of which is the hydrocarbon methane).

   • Coal mass is measured in tons and millions of tons.

   • The fuel used in nuclear power plants is manufactured from uranium oxide in the form of cylindrical pellets with a mass of 7 grams. Each of these pellets can provide as much energy as a ton of coal¹.

   • Finally, volumes of wood for fuel (biomass) are measured in terms of steres (st). This unit of measurement corresponds to one cubic meter of stacked logs, including the empty spaces between them. Biomass in the form of wafers or granules is expressed in terms of tons.

What Units Are Used to Measure Renewable Energy Sources?

   • Wind turbines convert the energy of the wind caused by its movement into electrical energy. A wind turbine starts to turn at a minimum wind speed of 4 meters per second (4m/s). The turbine's productivity (i.e. the amount of electricity it produces over a given period) doubles when wind speed reaches 7m/s. However, if wind speed increases to 25m/s, the turbine shuts down and doesn't produce any more energy (its blades are feathered, i.e. they reduce their angle to the wind). This is why the dominant winds of any proposed site are studied carefully before deciding build a wind turbine.

   • Similarly, hydroelectric plants have turbines that convert the energy released by flowing or cascading water into electricity. In every plant, the turbine's size is proportional to river flow, waterfall height or the altitude difference between the upper and lower reservoirs. River flow is measured in cubic meters per second (m³/s), while waterfall height or altitude difference is expressed in meters (m). Both of these factors are accounted for when calculating the electrical power produced by the plant, as is weight related to the earth's gravity, which comes into play in a waterfall. Thus, for a flow of 1m³/s, a hydraulic system produces 9.8 kilowatts (kW) of electricity per meter of waterfall. This data is crucial when determining where to set up a hydroelectric power plant.

   • Geothermal systems recover heat from the earth for domestic heating or for conversion into electricity. Before installing these systems, the heat in the subsurface of the potential site is taken into account. It increases by 3°C every 100 meters, but can vary significantly according to the location's geological features (this value is called the geothermal gradient).

   • Solar panels capture the sun's heat energy and convert it into heat to produce hot water or electricity. They recover most energy when they are perpendicular to the sun's rays. This is why their orientation is sudied carefully (south-facing panels capture more energy) as is their angle, expressed in degrees (°). However, the angle of the sun's rays varies from region to region, from season to season, and throughout the day.  All these factors must also be accounted for when finding the best orientation and angle for the panels. Watt-peak (Wp) is the unit that measures the maximum electrical power produced by a solar panel when correctly oriented and angled and exposed to a temperature of 25°C and sunshine of 1000 W per m². To express the maximum power produced by a solar power plant in the same conditions, a multiple of the watt-peak is used - the megawatt-peak (MWp) is equivalent to 1 million Wp.

Energy Production and Consumption- Measuring Electricity and Heat

Whatever the source used to obtain energy, the energy itself can be quantified using a number of units.

   • The joule (J) is part of the International System of Units. It is the universal unit for quantifying energy, whether in the form of work (the movement of a mass propelled by a force) or heat. From a physics standpoint, work and heat are the same. The joule is a very small unit that is not suitable for measuring large amounts of energy. This is why its multiples are more often used, such as gigajoule (GJ), where 41.855 GJ = 1 toe.

   • To measure a quantity of heat, the calorie (cal) is sometimes still used. This equates to 4.184 J and it is not part of the SI. A calorie is the quantity of heat required to raise the temperature of a gram of water by one degree Celsius (under normal atmospheric conditions of 1013.25 hPa and a starting temperature of 15°C). Heating engineers still use the thermie (th) - a multiple of the calorie, worth one million calories.

   • Similarly, the British Thermal Unit (BTU) is still used in the United Kingdom. This corresponds to the quantity of heat required to raise the temperature of a pound of water by 1 degree Fahrenheit (°F) - equivalent to heating 450g of water under normal atmospheric pressure of 1013.25 hPa at a starting temperature of 58.1°F (or 14.50°C). One BTU is equivalent to 1055 J.

   • Finally, the watt (W) is used to measure electrical power- i.e. the electrical energy produced per unit of time. One watt is equivalent to one joule per second. A kilowatt-hour (kWh) is used to assess a household's electrical consumption or an item of equipment. This unit is equivalent to the electrical power of 1 watt used for one hour, i.e. 3.6 MJ. Therefore, annual electricity consumption, excluding heating, for a family of four people is about 7,000 kWh².

[1] Source: CEA
[2] Source: Cité des Sciences et de l'Industrie